1. Field of the Invention
The present invention relates to an exhaust gas cleaning device for an internal combustion engine, and more particularly, the invention relates to the technology of recycling a particulate filter.
2. Description of the Related Art
Generally, there is increasing demand for the continual reduction of exhaust emissions from internal combustion engines used in automobiles and the like, and particularly in compression ignition type diesel engines that use diesel oil as fuel. CO, HC, and NOx are not the only gases desired to be removed from the exhaust gas, but also smoke and fine exhaust particles such as SOF contained in the exhaust gas. Therefore, a particulate filter is provided in the exhaust path to capture the fine particles contained in the exhaust gas.
The particulate filter lets an incoming exhaust gas pass through its porous diaphragm to capture fine particles present in the exhaust gas on the surface or fine pores of the diaphragm. As the quantity of particles to be captured and deposited increases to an excessive quantity, the back pressure in the internal combustion engine is raised as the flow resistance caused by the particulate filter increases. This decreases the performance of the engine. Therefore, fine exhaust particles captured by the particulate filter must be appropriately removed from the filter, so that the particulate filter can be recycled to restore its capability of capturing fine particles from the exhaust.
A known particulate filter includes an oxidation catalyst, such as platinum, provided in the filter enabling it to be recycled by the oxidation function of the catalyst during operation of the internal combustion engine. In this particulate filter, for example, post injection of fuel in the exhaust process supplies fuel to the particulate filter, so that the heat from the catalytically oxidized fuel is used to oxidize and remove deposited fine exhaust particles that are not oxidized as easily as the injected fuel.
Frequent recycling of the particulate filter degrades fuel efficiency while, if the recycling interval is too long, part of the excess of the deposited fine exhaust particles could abruptly burn, resulting in the particulate filter attaining abnormally high temperatures and becoming damaged. Therefore, the quantity of the deposited fine exhaust particles is preferably measured to determine when to start recycling. According to the disclosure of Japanese Patent Laid-Open Publication No. Hei 7-332065, the differential between the pressure at the inlet and at the outlet of the particulate filter is measured to allow a determination based on the fact that the differential pressure increases because of an increase in the flow resistance caused by the increased quantity of fine exhaust particles in the filter. Then, the point in time when the detected differential pressure is beyond a prescribed value is determined as the recycling time.
According to Japanese Patent Laid-Open Publication No. Hei 7-332065, however, the quantity of deposited fine exhaust particles could be different for the same detected differential pressure, depending on the operation state of the internal combustion engine. Accordingly, the quantity of deposited fine exhaust particles cannot necessarily be determined with sufficient accuracy. Detailed maps for different operation states could be stored, but cannot be easily provided for, as a large storage capacity would be necessary.
The invention is directed to a solution to the above-described disadvantage, and it is an object of the present invention to provide an exhaust gas cleaning device for an internal combustion engine in a simple structure that can properly determine the time of recycling the filter.
The inventors have researched and studied the particulate filter in connection with the deposition of fine exhaust particles and its effect upon the flow of the exhaust gas, and have found that the following model equation is established.
xcex94P=Mxcexcv+Nxcfx81v2
wherein xcex94P is the differential between the pressure at the inlet side and the outlet side of the particulate filter, i.e., the pressure loss in the particulate filter, v is the flow velocity of the exhaust gas flowing through the particulate filter, xcexc is the viscosity of the exhaust gas, and xcfx81 is the density of the exhaust gas. The coefficients M and N become larger as the quantity of the deposited fine exhaust particles becomes larger. More specifically, M or N is substantially a linear function of the deposited quantity ML, and when the deposited quantity is beyond a certain value, the ratio of change of M, N relative to the deposited quantity ML is reduced. The ratio changes before and after the certain value because the pressure loss in the particulate filter is initially increased according to the ratio of fine pores filled with fine exhaust particles to the fine pores in the particulate filter. Then, once almost all the pores are filled with fine exhaust particles, the pressure loss should change based on the thickness of the layer of deposited fine exhaust particles. The present invention is based on the findings.
According to a first aspect of the invention, an exhaust gas cleaning device for an internal combustion engine with a particulate filter provided in an exhaust path for capturing fine exhaust particles includes a differential pressure detecting means for detecting a differential between the pressure at the inlet side and the outlet side of the particulate filter, a flow velocity detecting means for detecting a flow velocity of an exhaust gas flowing through the particulate filter, and a recycling determining means for determining whether or not to recycle the particulate filter according to a determination formula for determining a value of a quantity of fine exhaust particles deposited in the particulate filter based on the detected differential pressure and flow velocity. The determination formula is equivalent to the following formula:
xcex94Pxe2x89xa7Mxcexcv+Nxcfx81v2
wherein xcex94P is the differential pressure, v is the flow velocity, xcexc is the viscosity of the exhaust gas flowing through the particulate filter, xcfx81 is the density of the exhaust gas, and M and N are constants. When the formula is satisfied, the particulate filter is allowed to be recycled.
As described above, in relation to the exhaust gas flow in the particulate filter, xcex94Pxe2x89xa7Mxcexcv+Nxcfx81v2 is established, and therefore when M and N are obtained for a reference deposited quantity, the value of the deposited quantity of the fine exhaust particles can accurately be determined based on the determination formula. In this way, the size of data to be stored is not as large as a map.
According to a second aspect of the invention, in the configuration according to the first aspect, the recycling determining means allows the deposited quantity to be calculated by the following determination formula:
ML=[xcex94Pxe2x88x92(Axcexcv+Cxcfx81v2)]/(Bxcexcv+Dxcfx81v2),
wherein ML is the deposited quantity, A, B, C, and D are constants, and the determination formula is used to compare the value of the calculated deposited quantity and a predetermined deposited quantity.
As described above, M and N are a linear function of the deposited quantity ML, and therefore the above calculation formula for ML is established in the model formula:
xcex94P=Mxcexcv+Nxcfx81v2
so that the deposited quantity of the fine exhaust particles is objectively available.
According to a third aspect of the invention, in the configuration according to the second aspect, the recycling determining means stores two calculation formulas for the deposited quantity having different values for the constants A, B, C, and D, one calculation formula is adapted to a deposited quantity in a range equal to or less than a predetermined reference value, while the other formula is adapted to a deposited quantity within a range equal to or more than the reference value, and when the deposited quantity calculated according to one calculation formula is out of the range of the deposited quantity to which the calculation formula is adapted, the deposited quantity is calculated again according to the other calculation formula.
When the chief cause for pressure loss in the particulate filter changes, a more appropriate determination formula is selected, and therefore the deposited quantity of the fine exhaust particles can be more accurately obtained.
According to a fourth aspect of the invention, in the configuration according to the second or third aspect, the particulate filter has an oxidation catalyst for oxidizing and burning the fine exhaust particles deposited therein. The device further includes a means for detecting reduction in the calculated deposited quantity during the recycling of the particulate filter, and a catalyst degradation determining means for determining the oxidation catalyst as being further degraded when the calculated deposited quantity is more slowly reduced.
When the oxidation catalyst is degraded and its oxidizing capability is lowered, the burning speed of the deposited fine exhaust particles is reduced during recycling of the particulate filter, so that the rate of reduction in the deposited quantity is lowered. As a result, whether or not the particulate filter is malfunctioning or degraded can be determined based on the reduction in the calculated deposited quantity.
According to a fifth aspect of the invention, in the configuration according to any one of the first to fourth aspects, the device further includes a temperature detecting means for detecting a temperature of the exhaust gas that flows through the particulate filter. The recycling determining means is set to obtain the viscosity xcexc based on the detected temperature according to a previously stored relation between the viscosity and the temperature of the exhaust gas.
Since the viscosity xcexc of the exhaust gas is more accurately obtained, a more appropriate determination formula can be provided.
According to a sixth aspect of the invention, in the configuration according to any one of the first to fifth aspects, the device includes a temperature detecting means for detecting a temperature of the exhaust gas that flows through the particulate filter. The recycling determining means is set to obtain the density xcfx81 based on the detected temperature according to a previously stored relation between the density and temperature of the exhaust gas. Since the density xcfx81 of the exhaust gas is more accurately obtained, a more appropriate determination formula can be provided.
According to a seventh aspect of the invention, in the configuration according to any one of the first to sixth aspects, the flow velocity detecting means includes an intake air quantity detecting means for detecting a quantity of air taken into the internal combustion engine, and a correcting means for adding a volume increase caused by combustion of injected fuel to an intake air quantity, thereby producing a volume flow rate of the exhaust gas that flows through the particulate filter. Then, the volume flow rate is converted into a flow velocity.
When injected fuel is burned in cylinders, the volume flow rate of the exhaust gas surpasses the intake air quantity depending on the quantity of fuel supplied for burning. Therefore, the volume flow rate of the exhaust gas can accurately be obtained without directly measuring the exhaust gas containing the fine exhaust particles. The flow velocity of the exhaust gas that flows through the particulate filter is proportionate through a coefficient defined by the shape of the particulate filter and can be converted from the volume flow rate.
When the flow rate of the exhaust gas containing the fine exhaust particles from the cylinders is directly detected, the detection characteristic could be affected by the contamination of the detection sensor with the fine exhaust particles. However, the flow rate of relatively clean intake air needs only to be detected, and therefore the detection characteristic is not affected, and highly reliable detection can be achieved for the flow velocity of the exhaust gas. Furthermore, an internal combustion engine is provided with an airflow meter that serves as a typical intake air detecting means, and the intake air quantity detected by the airflow meter can directly be used, in other words, the detection can be carried out in a simple structure.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.